Within the scope of this application, a process as defined above is described in the French Pat. No. 77 27 501. The surface of the strip is illuminated and, using a television camera, reflected images of successive portions of the surface of the strip are formed thus achieving a continuous representation of the inspected surface. Each image is scanned by comparing it at peak luminance level and minimum luminance level. A recording is made of the number of sensed defects on each image and of the percentage of the total area of the defects in relation to the area of the portion of the strip corresponding to an image.
It is known that inspection of rolled strips and especially hot-rolled strips at the exit of a pickling line is no easy matter, as, on the one hand, there is a large number of potential defects, varying more or less in size, visibility or consequence and, on the other, the surface of a strip is rough and exhibits spots due to uneven pickling. That is why, using the known prior art process described above, it is virtually impossible to achieve efficient sensing of all the actual defects without bringing in a substantial portion of overdetection.
Accordingly, in actual practice, a visual inspection of the strips is resorted to, so that the operator has a clear image of the strip in motion; the speed of the latter must necessarily be restricted. Now, it it desirable to carry out an inspection of both sides of the strip at high-speed and to sense the defects in real time. This capability would be very beneficial for making possible inspection of a strip after hot rolling and pickling and prior to cold rolling, when the cold-rolling strip mill is connected direct to the pickling line. It follows in such case that the combination of reliable sensing of unacceptable defects during the cold-rolling operation and of a fast moving rate, e.g., up to 6 m/s at the check station, is highly desirable.